Angiogenesis is the fundamental process by which new blood vessels are formed. The process involves the migration of vascular endothelial cells into tissue, followed by the condensation of such endothelial cells into vessels. Angiogenesis may be induced by an angiogenic agent or be the result of a natural condition. The process is essential to a variety of normal body activities, such as reproduction, development and wound repair. Although the process is not completely understood, it involves a complex interplay of molecules that stimulate and molecules that inhibit the growth and migration of endothelial cells, the primary cells of the capillary blood vessels. Under normal conditions, these molecules appear to maintain the microvasculature in a quiescent state (i.e., without capillary growth) for prolonged periods which can last for several years or even decades. The turnover time for an endothelial cell is about 1,000 days. Under appropriate conditions, however (e.g., during wound repair), these same cells can undergo rapid proliferation and turnover within a much shorter period, and five days is typical under these circumstances. (Folkman and Shing, J. Biol. Chem., 267(16), 10931-34 (1989); Folkman and Klagsbrun, Science, 235, 442-47 (1987)).
Although angiogenesis is a highly regulated process under normal conditions, many diseases (characterized as "angiogenic diseases") are driven by persistent unregulated angiogenesis. In such disease state, unregulated angiogenesis can either cause a particular disease directly or exacerbate an existing pathological condition. For example, ocular neovascularization has been implicated as the most common cause of blindness and underlies the pathology of approximately 20 eye diseases. In certain previously existing conditions such as arthritis, newly formed capillary blood vessels invade the joints and destroy cartilage. In diabetes, new capillaries formed in the retina invade the vitreous humor, causing bleeding and blindness.
Both the growth and metastasis of solid tumors are also angiogenesis-dependent (Folkman, J. Cancer Res., 46, 467-73 (1986); Folkman, J. Nat. Cancer Inst., 82, 4-6 (1989); Folkman et al., "Tumor Angiogenesis," Chapter 10, pp. 206-32, in The Molecular Basis of Cancer, Mendelsohn et al., eds. (W. B. Saunders, 1995)). It has been shown, for example, that tumors which enlarge to greater than 2 mm. in diameter must obtain their own blood supply and do so by inducing the growth of new capillary blood vessels. After these new blood vessels become embedded in the tumor, they provide nutrients and growth factors essential for tumor growth as well as a means for tumor cells to enter the circulation and metastasize to distant sites, such as liver, lung or bone (Weidner, New Eng. J. Med., 324(1), 1-8 (1991)). When used as drugs in tumor-bearing animals, natural inhibitors of angiogenesis can prevent the growth of small tumors (O'Reilly et al., O'Reilly et al., Cell, 79, 315-28 (1994)). Indeed, in some protocols, the application of such inhibitors leads to tumor regression and dormancy even after cessation of treatment (O'Reilly et al., Cell, 88, 277-85 (1997)). Moreover, supplying inhibitors of angiogenesis to certain tumors can potentiate their response to other therapeutic regimens (e.g., chemotherapy) (see, e.g., Teischer et al., Int. J. Cancer, 57, 920-25 (1994)).
Although several angiogenesis inhibitors are currently under development for use in treating angiogenic diseases (Gasparini, Eur. J. Cancer, 32A(14), 2379-85 (1996)), there are disadvantages associated with several of these proposed inhibitory compounds. For example, suramin is a potent angiogenesis inhibitor, but, at doses required to reach antitumor activity, causes severe systemic toxicity in humans. Other compounds, such as retinoids, interferons and antiestrogens appear safe for human use but have only a weak anti-angiogenic effect. Still other compounds may be difficult or costly to make. In view of these problems, there exists a need for methods and compositions for inhibiting angiogenesis.